Dipeptide mimics wherein the amide bond (--C(O)NH--) is replaced by a trans olefin ##STR1## are commonly employed as isosteric replacements for a dipeptide moiety in peptidyl enzyme inhibitors or in analogs of endogenous peptidyl ligands. Various methods have been disclosed for the preparation of such olefinic dipeptide isosteres. (See Shue, et al., Tet. Lett. 28 3225 (1987); Shue, et al., Tet Lett. 29 4041 (1988); Spaltenstein, et al., Tet. Lett. 27 2095 (1986); Spaltenstein, et al., J. Org. Chem. 52 3759 (1987); Hann, et al., J. Chem. Soc. Perkin I 307 (1982); Cox, et al., J. Chem. Soc. Chem. Comm. 799 (1980); Miles, et al., J. Chem. Soc. Perkin Trans. I 2299 (1985); Whitesell, et al., Chirality 1 89 (1989); Hanson, et al., J. Org. Chem. 50 5399 (1985)). However, there remains a need for a method for preparing olefinic dipeptide isosteres wherein the .alpha. and .delta. side chains can be introduced in a stereocontrolled manner.